Five get omen about poison (5)
I believe the answer is:
venom
'poison' is the definition.
(I know that venom is a type of animal toxin)
'five get omen about' is the wordplay.
'five' becomes 'v' (Roman numeral).
'get' indicates putting letters inside.
'about' indicates anagramming the letters.
'omen' with letters rearranged gives 'enom'.
'v' placed into 'enom' is 'VENOM'.
(Other definitions for venom that I've seen before include "Toxic substance" , "Poisonous fluid" , "Snake's poison" , "Spite: toxin" , "Virulent feeling" .)